The extragalactic γ-ray background: imprints from the physical properties and evolution of star-forming galaxy populations

Star-forming galaxies (SFGs) harbour an abundant reservoir of cosmic rays (CRs). At GeV energies, these CRs undergo interactions with their environment to produce γ-rays, and the unresolved γ-ray emission from populations of SFGs form a component of the isotropic extragalactic γ-ray background (EGB). In this work, we investigate the contribution to the 0.01-50 GeV EGB from SFG populations located up to redshift z = 3. We find this is dominated by starbursts, while the contribution from main-sequence SFGs is marginal at all energies. We also demonstrate that most of the γ-ray contribution from SFGs emanates from low-mass galaxies, with over 80 per cent of the emission originating from galaxies with stellar masses below $10^8 \, {\rm M}_{\odot }$. Many of these galaxies are located at relatively high redshift, with their peak EGB contribution arising ~700 Myr before the noon of cosmic star formation. We find that the precise redshift distributions of EGB sources at different energies imprint intensity signatures at different angular scales, which may allow their contribution to be distinguished using analyses of small-scale EGB intensity anisotropies, particularly if the diffuse EGB is dominated by hadronic CR-driven γ-ray emission from SFGs. We show that the EGB is sensitive to the evolution of low-mass populations of galaxies, particularly around z ~ 2.5, and that it provides a new means to probe the engagement of CRs in these galaxies before the high noon of cosmic star formation.